Why on earth would you want to do that? The TMY is such a beautiful film at box speed. If you want to use it at ISO 50, I second the opinion to use a TMX 100 speed film instead. That one actually works very well shot at EI 50 and then developed in Rodinal.

If you really, absolutely must pull film 3 stops, then I recommend a "stand developer." That would allow you to avoid blocked highlights, but at the cost of increased grain. If you try to use a regular developer (even Rodinal at 1:100), the developing time would be too short to accurately control -- shadows develop more slowly than highlights. Pulling any film (i.e., overexposing and then compensating by curtailed development) will not decrease the size of the grain as much as using a finer grained film to begin with. Indeed, because of the curtailed development, the apparent grain will be greater because the sulfite would not have had enough time to do its work.